The present invention relates to a cable, in particular a cable for electric power transmission or distribution or for telecommunications. In more detail, the present invention relates to a cable as above defined comprising at least one outer coating sheath and provided with self-repairing protection which is capable of restoring the continuity of the coating sheath after it has been broken.
Electrical cables, in particular low- or medium-voltage cables for the distribution of electric energy for domestic or industrial use, generally consist of one or more conductors individually insulated by a polymeric material and coated with a protective sheath, which is also made of a polymeric material. These cables, in particular when installed underground, either directly or inserted in tunnels or inside buried pipes, are subjected to damages on these layers caused by various types of mechanical abuses, for example accidental impact with sharp tools such as shovels or picks, which exert both cutting and compression actions on the cable, This can lead to partial or complete rupture of the outer sheath and possibly also of the inner insulating layer, which will bring about infiltration of moisture and generation of leakage currents. If rupture of the coating layers reaches the conductor, the combined effect of leakage currents and moisture leads to a gradual corrosion of the conductor until, at the most, a complete breakage of the conductor itself.
To obtain effective protection against such mechanical abuses, the cable can be provided with an outer structure capable of withstanding both cutting and compression, this outer structure consisting of a sheath made of a metal or a plastic material combined with a metal armouring, for example. In addition to being expensive, this solution leads to an important increase in the overall dimensions and rigidity of the cable, thus making this solution unsuitable for cables requiring easy installation and low costs, such as, in particular, in the case of low-voltage cables.
In Patent Application DE-1,590,958 a telecommunications or high-current cable is described which is protected from mechanical damages by means of an outer sheath provided, on its inside, with microcapsules containing a liquid that is capable of rapidly solidifying, once the microcapsule has been broken. To this purpose, use of the two components commonly employed for manufacturing expanded polyurethane is mentioned as the preferred one, these components being microencapsulated separately so that they react together on breaking of the microcapsules, forming an expanded material which closes the accidental cut. Alternatively, liquids solidifying when brought into contact with external agents, moisture for example, may be used.
According to the Applicant, the solution envisaged in the above-mentioned patent application is of difficult practical implementation and has many drawbacks. Firstly it is to note that the possibility of self-repairing is limited to the outer sheath, and no indications regarding the possibility of restoring integrity of the inner insulating layer are provided. In addition, to obtain an effective self-repairing effect, it is necessary to introduce a large amount of microencapsulated material during sheath extrusion, which operation can be rather difficult and also expensive. It is finally to be pointed out that the mechanism of action of the microcapsules is irreversible, so that the self-repairing effect can be carried out only once, i.e. at the moment the microcapsules are broken. Actually, during the various stages of the cable life (manufacturing, storage, installation, use), the coating layers are inevitably subjected to external mechanical actions of compression and bending and to thermal cycles of expansion and compression, which can lead to rupture of the microcapsules with consequent expansion and/or solidification of the material contained therein. This material therefore, will be no longer able to effect the desired self-repairing action when the sheath is actually damaged. It is also to be noted that, even when microcapsules are used which contain a liquid material solidifying on contact with moisture, accidental rupture of the microcapsules without any actual damage to the outer sheath leads in any event to solidification of the material because inside the cable there is always some residual moisture.
The Applicant has now found that, in consequence of a mechanical damage creating a discontinuity in at least one of the cable coating layers, it is possible to obtain effective self-repairing of the coating by virtue of the presence of an inner layer, placed, for example, between the insulating layer and the outer sheath, and comprising a material having a predetermined cohesiveness and at the same time a controlled flowability, which is capable of repairing the damage by restoring the continuity of the coating layer. After a discontinuity in the coating has been created, the material “moves” towards the damaged point and fills up the discontinuity at least partly by forming a substantially continuous layer which is capable of maintaining the cable functionality under the expected working conditions.
The action of the self-repairing material taking place with a reversible mechanism, among other things, prevents moisture infiltration and establishment of leakage currents, and consequently quick corrosion of the conductor.
Based on this starting perception, the Applicant has developed and set up a self-repairing cable and related manufacturing process, being the object of the Patent Application EP 99103092.5, contents of which is considered as herein reported for supplement and completion of the detailed description of the present invention as hereinafter set forth. In accordance with the present invention, the Applicant has now found that by arranging one or more anchoring portions between the outer sheath and the core of the cable, each housed in an interruption region of the self-repairing material extension, further improvements can be advantageously achieved in terms of cable reliability. In particular, any possibility of relative sliding between the outer sheath and inner core of the cable is advantageously eliminated, independently of whether said core is made up of one or more bare conductors or of conductors provided with one or more coating layers internal to the sheath.
In addition, also solved are problems resulting from unsteady positioning of the conductor within the self-repairing material bringing about off-setting of the conductor relative to the cable axis and thickness unevenness in the self-repairing layer itself.